1. Field of the Invention
The present invention relates to a method for stating up an insulated DC-DC converter which has a full-bridge configuration on a primary side of a transformer.
2. Description of Related Art
FIG. 9 shows an example of a DC-DC converter disclosed in Japanese Patent Application No. 2009-18302.
The DC-DC converter in FIG. 9 has a full-bridge configuration on a primary side of a transformer 9. MOSFETs are used as switching devices 1 to 4 which form a primary-side bridge. On the other hand, rectification devices 7 and 8 are used on a secondary side of the transformer 9. Capacitors 13 to 16 are parasitic capacitances of the switching devices 1 to 4 respectively, and capacitors 17 and 18 are parasitic capacitances of the rectification devices 7 and 8.
In FIG. 9, the portion of a gate drive circuit for generating gate signals is not shown. The gate signals serve as driving signals for switching on/off the switching devices 1 to 4.
Here, in the period when the switching devices 1 and 4 are ON concurrently, a current flows in a path from a DC power supply 11 through the switching device 1, a coil 5, the transformer 9 and the switching device 4 back to the DC power supply 11 on the primary side of the transformer 9 so as to apply a positive voltage to a primary-side voltage Vt1 of the transformer. On the other hand, in the period when the switching devices 2 and 3 are ON concurrently, a current flows in a path from the DC power supply 11 through the switching device 3, the transformer 9, the coil 5 and the switching device 2 back to the DC power supply 11 so as to apply a negative voltage to the voltage Vt1.
In this manner, a positive or negative voltage is applied to the primary side of the transformer 9, and the voltage corresponding to the turn ratio of the transformer 9 is generated on the secondary side and rectified by the rectification devices 7 and 8 so as to output a DC voltage in an output voltage Vo.
In the configuration of FIG. 9, the coil 5 may be replaced by leakage inductance of the transformer 9.
FIG. 10 is a chart showing changes of the gate signals serving as driving signals of the switching devices 1 to 4 and the voltage value Vt1 on the primary side of the transformer 9 when the DC-DC converter in FIG. 9 is started up.
When the DC-DC converter is started up by phase shift operation as shown in FIG. 10, the driving signals of the switching devices 1 to 4 have a duty ratio of 50% to switch on/off the switching devices 1 and 2 alternatively and switch on/off the switching devices 3 and 4 alternatively. In addition, the gates signals of the switching devices 3 and 4 are shifted in phase from the gate signals of the switching devices 1 and 2. Incidentally, dead times are provided between the gate signals of the switching devices 1 and 2 and between the gate signals of the switching devices 3 and 4 respectively so as not to turn ON those switching devices 1 and 2 or 3 and 4 concurrently.
Here, a positive voltage is applied to the voltage Vt1 in the period when the switching devices 1 and 4 are ON concurrently, and a negative voltage is applied to the voltage Vt1 in the period when the switching devices 2 and 3 are ON concurrently. Accordingly, when the phases of the gate signals of the switching devices 3 and 4 with respect to those of the driving signals of the switching devices 1 and 2 are adjusted, the pulse width of the voltage value Vt1 is varied so that the magnitude of the secondary-side output voltage Vo can be adjusted.
When the DC-DC converter is started up, the period when the switching devices 1 and 4 are ON concurrently and the period when the switching devices 2 and 3 are ON concurrently are shortened as shown in FIG. 10. In this state, the phases of the gate signals of the switching devices 3 and 4 are changed gradually to increase the period when the switching devices 1 and 4 are ON concurrently and the period when the switching devices 2 and 3 are ON concurrently, as shown in FIG. 11. As a result, the value of the output voltage Vo is increased from zero to a target voltage.
[Patent Document 1] Japanese Unexamined Patent Application No. 2009-18302
As described in Japanese Patent Application No. 2009-18302, when the aforementioned phase shift operation is performed under no-load or light-load conditions set between output terminals 19a and 19b, a switching device in an arm of the primary-side full bridge is turned on immediately after a switching device in the opposite arm thereto is turned off. Thus, the switching device in the arm is turned on when the switching device in the opposite arm thereto is near a zero voltage. Therefore, reverse recovery of the switching device on the opposite arm may occur.
When, for example, the switching device 2 is turned on immediately after the switching device 1 is turned off, there is a possibility that the switching device 2 may be turned on in the state where the voltage of the switching device 1 is zero. Since the voltage of the switching device 1 is zero, a current easily flows into a body diode of the switching device 1. Since the switching device 2 is turned on in this state, reverse recovery of the body diode of the switching device 1 occurs. Typically, when the voltage-time change rate dv/dt at the reverse recovery of the body diode is beyond its maximum rated value, there is a fear that a MOSFET constituting the switching device is broken. Thus, occurrence of the reverse recovery increases loss and lowers the reliability of the apparatus remarkably. On the other hand, when the DC-DC converter is started up by PWM (Plus Width Modulation) operation, the duty ratio of each gate signal is lower than 50% as shown in FIG. 12. It is therefore possible to secure a long enough period in which the opposite switching devices in the upper and lower arms are OFF concurrently. Thus, sufficient time to increase a voltage can be secured after the switching devices are OFF. A current can be prevented from flowing into any body diode and therefore reverse recovery can be prevented from occurring.
However, to stabilize a forward bias voltage and a reverse bias voltage even if the duty ratio of an input voltage to a transformer is not about 50%, by use of a gate drive circuit which is possible to apply a reverse bias voltage between a gate and a source, the reverse bias voltage cannot be obtained if the pulse width in each gate signal is not wide enough.
When, for example, the DC-DC converter is started up, the switching devices 1 and 4 and the switching devices 2 and 3 must be driven by gate signals each having a narrow pulse. In the gate drive circuit at that time, however, a capacitor provided on the secondary side of the transformer is charged only in a period when the switching devices are ON. Thus, the capacitor cannot provide a sufficient reverse bias voltage Vr.
As a result, there is a fear that the switching devices 1 to 4 can be turned on due to induced noise from the outside, or the like, to thereby result in reduction of the reliability of the converter.
When the DC-DC converter is started up by gate signals each having a pulse width expanded in advance as shown in FIG. 13 in order to avoid the fear of switching devices 1 to 4 being turned on by induced noise, there is, however, another fear that the output voltage Vo may surpass the withstanding voltage of an apparatus connected thereto and therefore the apparatus may be broken. When, for example, the switching devices 1 and 4 are turned on concurrently, a positive voltage is applied to the voltage Vt1. Here, the voltages of the parasitic capacitances 17 and 18 are zero, and hence the voltage of each winding of the transformer 9 is also zero. Thus, the input voltage of the DC power supply 11 is applied to the coil 5 to suddenly increase the current flowing into the coil 5. Then, the energy stored in the coil 5 moves into the parasitic capacitance 18 due to the resonance operation between the coil 5 and the parasitic capacitance 18. As a result, the voltage of the parasitic capacitance 18 increases up to twice as large as the secondary-side voltage of the transformer 9. Then, the energy stored in the parasitic capacitance 18 moves into an output capacitor 12 through the secondary-side of the transformer 9 and a DC reactor 10. Thus, the output voltage Vo increases largely.
Typically there is no load when the DC-DC converter is started up. It is therefore impossible to discharge the energy stored in the output capacitor 12. Accordingly, when a switching operation is repeated on the switching devices 1 to 4 after the DC-DC converter is started up with the gate signals whose pulse widths are expanded as shown in FIG. 13, the output voltage Vo increases beyond the target voltage as shown in FIG. 14. As a result, secondary-side components of the DC-DC converter or an apparatus connected to the load may be broken.
In order to avoid this problem of high output voltages harming components, high-voltage components are typically used in the secondary-side components and the apparatus connected to the load, thereby causing increase in cost and loss.